In the prior art, it is known to filter and combine signals from two or more spatially separated microphones to obtain a directional microphone signal. This form of signal processing is generally known as beamforming. The quality of beamformed microphone signals depends on the individual microphones having equal sensitivity characteristics across the relevant frequency range, which, however, is challenged by finite production tolerances and variations in aging of components. The prior art therefore comprises various techniques directed to calibrate microphones or otherwise handle deviating microphone characteristics in beamformers.
European patent application EP 2884763 A1 discloses a headset with a microphone apparatus adapted to provide an output audio signal (O) in dependence on voice sound received from a user of the microphone apparatus, where the microphone apparatus comprises a first microphone unit (M1) adapted to provide a first input audio signal in dependence on sound received at a first sound inlet and a second microphone unit (M2) adapted to provide a second input audio signal in dependence on sound received at a second sound inlet spatially separated from the first sound inlet (see FIG. 1 and paragraphs [0058]-[0065]). The microphone apparatus further comprises a linear main filter with a main transfer function adapted to provide a main filtered audio signal in dependence on the second input audio signal, a linear main mixer (BF1L) adapted to provide an output audio signal (XL) as a beamformed signal in dependence on the first input audio signal and the main filtered audio signal, and a main filter controller adapted to control the main transfer function to increase the relative amount of voice sound in the output audio signal (O) (see FIG. 1 and paragraphs [0066]-[0069]). It further suggests “ . . . using microphones with very small variations in sensitivities . . . ” or “ . . . microphone sensitivities may be estimated in a calibration step at the time of production.” to ensure equal sensitivity characteristics. Both of these measures would normally increase production costs.
Also, adaptive alignment of the beam of a beamformer to varying locations of a target sound source is known in the art. There is, however, still a need for improvement.